The present invention relates to a frequency separator for use in branching microwaves, specifically relates to a frequency separator for use in branching bands for dual-band handy phones.
FIG. 10 is a schematic diagram showing a circuit of a conventional frequency separator which comprises a low-pass filter and a high-pass filter. FIGS. 11 and 12 are block diagrams showing other conventional frequency separators disclosed in Japanese Patent Laid-Open No. 3-216002. The frequency separator of FIG. 11 corresponds to that of FIG. 10 which has an additional low-pass filter connected to the high-pass filter. The frequency separator of FIG. 12 corresponds to that of FIG. 11 in which a rejection filter is added to each of the low-pass filter and the high-pass filter.
Generally, these frequency separators have a dielectric coaxial resonator which comprises a dielectric ceramic cylinder having electrodes formed on both the inner and outer surface of the cylinder. Due to this structure, it has been difficult to reduce the size of the frequency separator.
The recent spread of the handy phone is striking, and many efforts are directed to improve the performance of portable terminal equipment. For example, a dual-band handy phone, which enables speech in two different frequency bands, has been proposed. The dual-band handy phone must have a device for selecting one from the two frequency bands, i.e., a frequency separator. Also, such a frequency separator must be small in size to be mounted in a portable terminal equipment.
FIG. 13 is a graph showing insertion loss of a frequency separator, having a conventional branching circuit shown in FIG. 10, for use in a dual-band telephone using two bands of f1=824 to 894 MHz and f2=1850 to 1990 MHz. As seen from FIG. 13, the attenuation is as small as 5 to 10 dB in both bands, and therefore, little amount of one band signal does not run around into the other band. Thus, the conventional frequency separator has been insufficient in branching one band signal f1 and the other band signal f2.
By the constructions shown in FIGS. 11 and 12, the insertion loss can be decreased with respect to each of the f1 and f2 bands. However, these constructions are rather complicated, and therefore, frequency separators having such constructions are inevitably large in size. Since the frequency separator to be mounted in a handy phone is required to be as small in size as possible and be of high performance, the conventional frequency separator of large size is apparently not suitable for a handy phone.